1. Field of the Invention
The invention is an infrastructure function needed in large scale wireless local or premises area networks where the user Stations are or may include battery-powered portable computers and pocket telephones either fixed or moving. The service function is setup and low-delay transfer of either or both packet data or virtual connections by means of limited length segmental packet transmission. The system architecture providing these functions is the subject invention. Microwave radio frequencies are assumed to be the primary transmission means, however optical propagation is also a usable medium.
2. Description of Prior Art
If many common channel wireless Access-points are placed sufficiently close together to obtain near continuous coverage over a wide area, then the communication between one Station and the nearest Access-point may be subject to interference from simultaneous activity at other nearby Access-points.
The commonplace and trivial solution is to reduce the total traffic carried in the system to a point where collisions are improbable, and then to provide a recovery mechanism when they do occur. This is the philosophy when an effort is made to adapt the IEEE 802.3 CSMA/CD access method to wireless (as discussed in Rypinski U.S. Pat. No. 5,461,627).
This difficulty can be resolved by using Access-points sequentially, rather than simultaneously, within a group pattern. There remains the obvious problem of common control and routing, and two non-obvious problems:
1) making movement of Stations between Access-point coverages invisible to external interconnected networks, and PA1 2) making the speed of adaptation to changed access path less than the smallest inter-message time spacing possibly only a few milliseconds. PA1 1) the means of dealing with Stations that move between Access-points during the potentially small (milliseconds) time interval between consecutive segments or messages; PA1 2) the architecture of a Hub Controller common to many Access-points which provides this function; and PA1 3) a means of coordination of the sequential pattern among contiguous groups of patterns. PA1 1) the algorithms executed in the hub common control function; and PA1 2) the architecture of the Hub Controller. PA1 1) Synchronized Sequential Scan; PA1 2) Synchronized Sequential Scan with Adaptive Stepping Time; PA1 3) Synchronized Sequential Scan with Adaptive Stepping Time and Cumulative Opportunity Window; and PA1 4) Adaptive Unsynchronized Sequential Scan.
Bridge-per-Access-point Architecture
Within the IEEE 802.11 Standards Committee and in other forums for wireless local area networks, it has been suggested that each wireless Access-point be a tap on a common backbone local area network (LAN). The backbone LAN, for example, might be: IEEE 802.3 CSMA/CD (Carrier-Sensing Multiple Access/Collision Detecting). This LAN in one version uses "daisy-chained" coaxial cable and in another version telephone pairs as the connecting physical medium, where these pairs are installed between each Station and a common hub unit.
Each tap on a backbone LAN is a bridge or router to an interconnected network depending on the protocol level at which the interconnection is made. Bridges have "filters" so that the bridge does not pass messages between networks which are local in either network alone. Routers have the capacity to direct an incoming message on one network to another bridge on another network or to select between a plurality of connected networks for forwarding. A gateway may do all of these things, but is used where the connected networks are of different types.
Inter-network Routing
To facilitate routing, automatic functions have been defined. The first of these is called "spanning tree" using an algorithm defined in IEEE Local Area Network Standard 802.1D "Media Access Control (MAC) Bridges." Only a few points in this complex area must now be understood.
The bridge depends upon tables identifying the network with which various addresses are associated. If the network is reached from a particular bridge through intermediate bridges, then only the next relaying bridge is known. All of this information is "learned" when the bridge listens to its ports, and when it is asked to relay a message to a new destination. In this process, exploratory messages may be generated to determine routing to a new address.
An event occurs when a new Station appears (or disappears) or when a Station addresses another which is not presently known on a connected network. Such events may cause many exploratory messages and responses to update bridge filtering and routing tables.
If each Access-point is bridged into a common backbone LAN, such events will occur whenever a Station changes from the coverage of one to another Access-point. This may occur from a movement of a few feet or from passing obstacles like walking persons. The smaller the coverage of each Access-point, the greater the frequency of coverage changes for comparably moving Stations.
The philosophy of bridging in LAN is that each Station is normally on one network and infrequently (in terms of radio fading) moves to another. The reconfiguring messages take time, though not much by human reference. Many continuing moves by many Stations create the possibility of overloading the backbone network with learning tasks.
Efficiency
A bridging between an 802.11 and an outside LAN may have much more function to support routing than does bridging between two 802.11 LAN access points since the same function in the Hub Controller is common equipment. A further consideration with bridge-per-Access-point configurations is that within a sequential group only one transmitter at a time is used. There is no way to avoid provisioning of transmit medium access control and other functions at all Access-points.
Selection Diversity
Prior art in more conventional radio systems uses duplicate receiving systems each connected to an antenna separated from the others but all at a common site. If the received signal is continuous, a switch is used to select the output of the receiver with the best signal. If the signal is bursty, then the selection decision is made within a very short interval after the signal appears. More refined versions would base the selection on signal-to-noise ratio rather than signal level.
This prior art is used for voice communication, and is not very relevant to data burst transmission. Diversity systems which sum the signals from several antennas before or during demodulation are entirely irrelevant to this problem. Finally, multiple antennas at one site is not the same problem as selecting between signals from one of several sites.
Coordination of Activity Among Large Numbers of Base Stations
Many prior art systems are frequency-division channelized; and some provide time-division sub-channels to increase the communication capacity at one base station. "Cellular" mobile telephone is based on a "reuse" group size. Systems are planned on the basis that 7, 9, 12 or more channel groups are available for simultaneous use when contiguously located. The limits are determined by the geographic spacing necessary for independent operation of the same channel at different places consistent with continuous coverage on one or another channel at nearly all places.
Considering "reuse" factor, the coordination between reuse groups is not known to have been addressed in any other context but cellular wireless telephony or its proposed successors. Even there, the considerations in a channelized system are quite different than for time separation.